EP 1165-2-1
30 Jul 99
flood of a given frequency it will always do so in the future or vice
versa, assuming the levee has been repaired.
(4)
Reliability.
(a) Reliability judgments should be based solely on physical
phenomena. The question to be answered is: what percent of the time
will a given levee withstand water at height "x"? This means that
considerations such as degree of protection, induced damages, induced
flood heights, potential for increased risk of loss of life due to
false sense of security, etc., are not included. These considerations
will be dealt with separately during the plan formulation process.
(b) The purpose of reliability determination is to be able to
estimate the without-project damages. Its purpose is not to make
statements about the degree of protection afforded by the existing
levees. Major subordinate commands (MSC) and district commands (DC)
making reliability determinations should gather information to enable
them to identify two points on the existing levees. The first point
is the highest vertical elevation on the levee such that it is highly
likely that the levee would not fail if the water surface elevation
were to reach this level. This point shall be referred to as the
Probable Non-failure Point (PNP). The second point is the lowest
vertical elevation on the levee such that it is highly likely that the
levee would fail. This point shall be referred to as the Probable
Failure Point (PFP). As used here, "highly likely" means 85+ percent
confidence. As defined, the PNP will be at a lower elevation than the
the PFP. When there are unresolved uncertainties or differences of
opinion, consideration should be given to having the range of
uncertainty extend from the lower of arguable PNPs to the higher of
the PFPs. Because of lack of information or other reasons, if the PFP
cannot be determined then the PFP shall be the low point in the levee
where the levee is first overtopped. When determining the low point
in the levee, MSC and DC shall assume that closure actions have taken
place.
(5) Benefit Evaluation Procedure. Even if no degree of
protection is claimed for an existing levee, it does, most likely,
provide some benefits. Assessment of these benefits must be in some
degree arbitrary in the absence of illuminating engineering or
statistical analyses. The function of identifying the probable
failure and non-failure points is to create a range of water surface
elevations on the levee over which it may be presumed that the
probability of levee failure increases as water height increases. The
requirement that as the water surface height increases the probability
of failure increases, incorporates the reasonable assumption that as
the levee becomes more and more stressed it is more and more likely to
fail. If the form of the probability distribution is not known, a
linear relationship is an acceptable approach for calculating the
benefits associated with the existing levees. For benefit evaluation,
assume all flood damages will be prevented below the PNP; and no
damages will be prevented above the PFP.
f. Restoration of Market Values. Valid estimates of restored
market value are difficult and costly to make in typical flood control
project evaluations. Therefore, no resources should be used in
efforts to quantify restoration of market values for flood control
projects.
13-15.
Flood Emergency Operations and Disaster Assistance.
13-13
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